According to a conventional automatic transmission housing friction engagement elements such as a friction clutch and a friction brake, the friction engagement elements are hydraulically controlled so as to be engaged or disengaged by use of an accumulator and an orifice. However, somewhat recent developments have led to an automatic transmission in which an oil pressure from an oil pressure source is directly controlled by use of a solenoid valve, thereby capable of controlling the oil pressure to be supplied to each friction engagement element without the use of the accumulator and so on. In such a case, a smooth and highly responsive shift operation can be performed in the automatic transmission and a driver can feel a comfortable shift feeling.
In the above-described direct control of the oil pressure to be supplied to each friction engagement element, meanwhile, the oil pressure control of the friction engagement element, i.e. a control of a piston stroke is still demanded to be performed with an improved response at the start of engaging the friction engagement element. Generally speaking, the friction engagement element is filled in with oil at a rapid speed corresponding to increase of the oil introduced thereto at the start of the engaging operation. This type of oil pressure control is so-called pre-charge control. In this case, the piston disposed in the friction engagement element can be hereby moved with the aid of the oil pressure of the oil rapidly filling the friction engagement element, so that the control of the piston stroke can be effectively prevented from being delayed.
The oil introduced into the friction engagement element is then reduced after the pre-charge control and the oil pressure therein is maintained at a relatively low oil pressure (i.e. a standby pressure), which approximately corresponds to a biasing force of a return spring disposed in the friction engagement element, for a predetermined time. Accordingly, surge pressure can be prevented from being generated at a piston stroke end.
An oil pressure characteristic value upon the pre-charge control sometimes may fluctuate due to an individual difference of an automatic transmission, an engine, or the like. In order to solve the aforementioned problem, an oil pressure control device disclosed in a Japanese Patent Laid-Open published as No. 07(1995)-27212 calculates an oil pressure control amount based upon a data base (a map) established by an experiment and the like.
The piston stroke control can be optimized in a short or long term by learning the piston stroke. According to a control system for an automatic transmission disclosed in a U.S. Patent as U.S. Pat. No. 5,813,943, a biasing force of a return spring disposed in a clutch C1 (a start clutch), i.e. a standby pressure can be estimated by optimally performing a neutral control in the automatic transmission.
When the automatic transmission, an engine, an electronic control unit, and the solenoid valve (an oil pressure control unit) have been mounted on a vehicle at a delivery initial condition, it may be difficult to comprehend an entire individual difference so that the oil pressure characteristic value for the friction engagement element may not be accurately calculated or estimated at the start of engagement thereof by the foregoing conventional oil pressure control device.
The present invention therefore seeks to provide an improved method for determining an oil pressure characteristic value by an automatic transmission system, by which the entire individual difference at the delivery initial condition can be absorbed.